Why does the hummingbird live with the ocotillo?

The Ocotillo shrub is a crucial supplier of food for hummingbirds during their migration north to the mountain regions of the western US from Mexico. The hummingbirds eat the nectar from the bright red flowers that populate the shrub during the same time as their annual migration. For mysterious reasons, hummingbirds seem to prefer red flowers, and the ocotillo fills the bill (pun intended).